The use of security threads in security documents, particularly bank notes, has become widespread over many years. For example the presence of a security thread is one of the most well recognised security features in United Kingdom currency. Security documents containing security threads have developed with ever increasing levels of sophistication. For example such threads may be provided with a repeating printed pattern of characters along their length, or may contain regions of magnetic material for machine-readable detection. One of the most striking visual developments of such threads is in the concept of a “windowed” thread in which windows are formed in the document substrate during its manufacture so as to expose the thread embedded within the document. The regions between the windows are known as bridges and in these regions the substrate material is formed over the thread such that the thread remains buried.
A significant security benefit would be gained if indicia on a windowed security thread could be positioned in register with the windowed areas. Moreover, such registration necessarily means that images may also be concealed under the paper bridges and provide features that may only be visible in transmission.
To date, such registration capability remains impractical since current production processes do not permit threads to be registered to achieve these desired effects. Instead, an arbitrary offset occurs between the bridge positions and thread design, giving rise to ill-centred or incomplete images in the window.
Not surprisingly attempts have been made to adjust the relative position of the thread to the window during the paper making process. Most notably trials have been undertaken to adjust the tension on the thread-feed during the paper making process. These have experimented with a closed loop feedback architecture, where a control signal is derived from an image capture system monitoring a selected window position. However, in practice, this approach has proved to be particularly difficult to implement, and little success has been achieved for anything other than particularly narrow threads.
The desire to achieve registration between images upon threads and the windows of the substrate remains strong. Most broadly, the goal is to find a method to facilitate the arbitrary positioning of images and motifs relative to the thread window, or (if required) the positioning of arbitrary and differing designs in each window. Furthermore an additional challenge is faced in that, in practice, such a method would need to integrate readily with the normal established processes of security document production, not least since such processes are required to meet the most exacting of production standards.